1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus including a developing unit operable with a one- or two-ingredient type developer and, more particularly, to a device for replenishing toner to the developing unit.
2. Discussion of the Background
In a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus, a latent image is electrostatically formed on a photoconductive element or image carrier and then developed by toner fed from a developing unit to turn out a toner image. The toner image is transferred from the photoconductive element to a paper or similar recording medium and then fixed by a fixing unit. Because the toner is sequentially consumed by repeated development, a toner replenishing device replenishes toner to the developing unit in order to make up for the decrease in the toner content of the developer. This allows a preselected toner content to be stably maintained.
For an image forming apparatus of the type consuming a relatively small amount of toner, i.e., Producing a relatively small number of copies, the toner replenishment from the above replenishing device suffices. However, an image forming apparatus of the type producing a relatively great number of copies or often using papers of relatively great sizes consumes a great amount of toner. With this type of apparatus, therefore, it is necessary to replace a toner bottle or cartridge frequently, wasting time and labor.
In light of the above, it has been proposed to increase the capacity, i.e., size of the toner bottle or cartridge. However, an increase in the size of the toner bottle directly translates into an increase in the overall size of the apparatus, and must therefore be restricted. Further, a large size toner bottle is difficult to rotate unless a considerable torque is applied thereto, obstructing toner replenishment. While the rotation of the toner bottle for driving the toner toward its toner outlet may be replaced with a pump or the like, such an alternative scheme increases the cost.
On the other hand, the large size toner bottle or cartridge may be replaced with a plurality of toner bottles or cartridges, as proposed in the past. Japanese Patent Laid-Open Publication No. 2-277083, for example, discloses a toner replenishing mechanism including a toner server accommodating a plurality of toner cartridges. The toner cartridges are automatically switched a plurality of times so as to reduce the frequency of replacement.
Japanese Patent Laid-Open Publication No. 4-115273 teaches an image forming apparatus including a cartridge storing device storing a plurality of toner cartridges. The cartridge storing device automatically feeds toner from a particular one of the cartridges to a toner replenishing device while collecting an emptied toner cartridge. Specifically, after an emptied toner cartridge is retracted from the toner replenishing device, a new toner cartridge is brought to the replenishing device. Subsequently, the empty cartridge is pulled out of the cartridge storing device. With this kind of cartridge storing device, it is possible to continuously feed the toner to the developing unit without interrupting the operation of the apparatus.
However, a problem with the above conventional schemes is that a plurality of toner cartridges are selected at random, preventing the toner server body or the cartridge storing device from having a uniform service life. That is, each cartridge storing portion must be provided with a particular service life and managed independently of the others, resulting in troublesome management. Another problem is that the toner of the cartridge which will be used later looses fluidity and cannot be easily driven toward the mouth of the cartridge at the time of replenishment. This aggravates the amount of toner to be left in the cartridge. In addition, the toner with deteriorated fluidity deposits on the inner wall of the cartridge, further aggravating the amount of toner to be left in the cartridge.
The toner server taught in the above Laid-Open Publication No. 2-277083 must be located next to the developing unit, hindering miniaturization of the apparatus. Moreover, the toner server has a sophisticated structure and cannot be maintained with ease.